Configurable, Power Supply Voltage Referenced Single-Ended Signaling with ESD Protection

ABSTRACT

A single-ended data transmission system transmits a signal having a signal voltage that is referenced to a power supply voltage and that swings above and below the power supply voltage. The power supply voltage is coupled to a power supply rail that also serves as a signal return path. The signal voltage is derived from two signal supply voltages generated by a pair of charge pumps that draw substantially same amount of current from a power supply.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/138,077 filed on Apr. 25, 2016, which is a continuation of U.S.patent application Ser. No. 14/072,307 filed on Nov. 5, 2013, which is acontinuation of U.S. patent application Ser. No. 13/318,475 entered intothe U.S. on Nov. 1, 2011, which is a National Stage of InternationalApplication No. PCT/US2010/029101 filed on Mar. 29, 2010, which claimsthe benefit of U.S. Provisional Application No. 61/176,411 filed on May7, 2009, the contents of which are each incorporated by reference.

BACKGROUND

The present disclosure relates to single-ended signaling.

Single-ended signaling is commonly used to transmit electrical signalsover wires, such as those in contemporary memory systems. Insingle-ended signaling, a signal represented by a varying voltage istransmitted over a wire. The voltage varies above or below a referencevoltage, which is used by a receiver of the signal to determine thedigital values represented by the signal. In conventional single-endedsignaling, the reference voltage is typically generated, either on-chipor off-chip, to be at the middle of a signal swing.

Conventional single-ended signaling typically has poor efficiency andlimited speed. More specifically, conventional single-ended signalinghas the disadvantage that (i) the reference voltage has to be generatedfor the receiver, (ii) it is difficult to match the reference voltage atthe receiver with the reference voltage of the transmitter, (iii) noisesuch as simultaneous switching noise can be generated due to the use ofa power supply voltage for signaling, and (iv) it is difficult tocontrol a signal return path, resulting in crosstalk noise.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure herein is illustrated by way of example and withoutlimitation in the figures of the accompanying drawings and in which likereference numerals refer to similar elements and in which:

FIGS. 1A, 1B, 1C, and 1D illustrate a data transmission system employingpower supply voltage referenced single-ended signaling, according to oneembodiment.

FIGS. 1E and 1F illustrate a cutaway view of an IC package including anintegrated circuit of the data transmission system of FIGS. 1A through1D on a printed circuit board.

FIGS. 2A, 2B, 2C, 2D, 2E, and 2F illustrate embodiments of signalvoltage supplies employed by the data transmission system of FIGS. 1Athrough 1D.

FIGS. 3A and 3B illustrate receiver amplifiers suitable for use in thedata transmission system of FIGS. 1A through 1D, according to oneembodiment.

FIG. 3C is an alternative embodiment of a common-gate amplifier of FIG.3B.

FIG. 4A illustrates a data transmission system employingground-referenced single-ended signaling with ESD protection, accordingto one embodiment.

FIG. 4B illustrates a voltage swing of a signal in the data transmissionsystem of FIG. 4A, according to one embodiment.

FIG. 5A illustrates a configurable data transmission system configuredin a single-ended signaling mode, according to one embodiment.

FIG. 5B illustrates a configurable data transmission system configuredin a differential signaling mode, according to one embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

The Figures and the following description relate to embodiments by wayof illustration only. Alternative embodiments of the structures andmethods disclosed herein may be employed without departing from theprinciples of the present disclosure.

Embodiments of the present disclosure include a data transmission systemusing a single-ended signaling system in which a signaling voltage isreferenced to a reference voltage that is a power supply voltage sharedby a transmitter and a receiver. The signaling voltage swings above andbelow the reference voltage. The reference voltage may be groundvoltage, generally taken to be the lowest power supply potential. Thesignaling voltage is derived from a first signal supply voltage and asecond signal supply voltage, which may be generated by a pair of chargepumps that draw substantially the same amount of current from a powersupply, regardless of the output drive level of the transmitter.

FIGS. 1A, 1B, 1C, and 1D illustrate a data transmission system 100employing supply voltage referenced single-ended signaling, according toone embodiment. The data transmission system includes first and secondintegrated circuit devices (ICs) 100 a and 100 b coupled to each othervia a plurality of conducting lines including signal line 132. ICs 100 aand 100 b may be separate devices with separate IC packages, or combinedwithin a die-stack or other multi-die package, or implemented by baredie. In the case of a multi-die package, the conducting lines may beimplemented by intra-package conductors such as wire-bonds, flexconductors or any other suitable interconnection structures. In the caseof separately packaged ICs or bare die, the conducting lines may beimplemented partly by conductive structures for conveying signals (andestablishing supply-rail connection) through the package to an externalinterconnect, and partly by printed circuit board (PCB) traces, PCBsupply bus layers, flex cables or other conductive interconnects betweenthe IC devices. With regard to system function, the ICs may be, forexample and without limitation, a processor and chipset (or bridge orother application-specific IC (ASIC)), memory controller and memorydevice, or any other pair of ICs that carry out operations involvingchip-to-chip signaling.

Still referring to FIG. 1A, IC device 100 a is to be connected to apower supply. IC device 100 a includes a transmitter 102 and terminals(such as pads or pins) 133, 134. Transmitter 102 is coupled between twosignal supply voltages V1 and V2, which are provided by one or morevoltage sources 140. In one embodiment, V1 and V2 are substantiallysymmetrical with respect to a power supply voltage Vp associated withthe power supply (i.e., V1−Vp=Vp−V2, at least approximately). Terminal134 is coupled to a power supply rail 130 which is coupled to powersupply voltage Vp, while terminal 133 is coupled to signal line 132. Bythis arrangement, when input data 106 is applied to transmitter 102, asignal that swings between about ½V1 and about ½V2 according to theinput data 106 is output onto signal line 132 via terminal 133. Thus,transmitter 102 forms a voltage mode transmitter that generates a datasignal 120 having a signal voltage that swings substantiallysymmetrically about the power supply voltage Vp (i.e., between about ½V1and about ½V2) to convey the bits of the input data 106.

Because the transmitted signal swings symmetrically about the powersupply voltage Vp, the power supply voltage may be used as a signalingreference voltage. In the embodiment shown, for example, the signal line132 and the power supply rail 134 are coupled, via terminals 136 and138, to respective inputs of a receiver 104 in circuit 100 b. Receiver104 amplifies the time-varying difference between the signal voltage andthe power supply voltage to produce the received data signal (RData)that corresponds to the originally transmitted data 106. That is,receiver 104 compares the signal received via terminal 136 with thereference voltage supplied via terminal 138 to recover the transmittedsignal and generate RData. For example, RData is “1” when the signalingvoltage at terminal 136 is higher than the reference voltage at terminal138, and RData is “0” when the signaling voltage at terminal 136 islower than the reference voltage at terminal 138.

Bi-directional signaling may be effected by including in IC device 100 ba transmitter 102′ similar to transmitter 102 and in IC device 100 a areceiver 104′ similar to receiver 104. The reverse-directiontransmitter/receiver 102′/104′ may use the same signaling line 132and/or the same supply-rail signaling reference 130 astransmitter/receiver 102/104 (thus driving signaling line 132bi-directionally). Or, the reverse-direction transmitter/receiver102′/104′ may drive a separate signal line, and/or employ a separate ordifferent supply-rail signaling reference. Also, while not specificallyshown within FIG. 1A, a mesochronous or plesiochronous timingarrangement may be used to enable synchronous data transmission andreception over link 132. For example, a clock signal may be provided toboth IC devices 100 a and 100 b (or forwarded from one device to theother) to provide timing for delivery of the input data 106 to thetransmitter 102, and to provide timing for latching of the output ofreceiver 104 within an input data register or like storage circuit.Alternatively (or additionally), a source synchronous timing reference(e.g., a strobe signal that transitions to mark the presence of data onthe signaling line 132) may be output from the transmitting IC to thereceiving IC to control the signal sampling time within the receivingIC. Various timing vernier circuitry may be provided and calibrated toestablish data output timing and/or data sample timing within desiredmargins.

In one embodiment, the V1/V2 signaling supply voltages are selected toeffect a small signal swing Vs (e.g., Vs≈½(V1−V2) is substantially lowerthan corresponding on-chip logic levels) and thereby enable low-powersignaling. The signaling supply voltages may be driven to programmablyselected or designed setpoint voltages (as discussed below), or may beadaptively established or calibrated to achieve a threshold operatingmargin as determined by bit error rate or other signal quality metric.

Charge-pump-based voltage supplies or any other supply voltage circuitsor voltage regulators may be used to implement the one or more voltagesources 140, as discussed in more detail below. In one embodiment, forexample, the voltage source(s) 140 is designed such that signal supplyvoltages are regulated and the net current drawn by transmitter 102 fromthe power supply remains substantially constant regardless of the outputvoltage level at terminal 133, thereby reducing switching noise withinthe signaling system.

The signaling arrangement of FIG. 1A provides a number of benefitsrelative to conventional approaches. First, the power supply rail 130may be implemented as a continuous conducting plane (e.g., power plane)that underlies all or most of the signal lines within an IC package orPCB, and thus constitutes a low and sometimes lowest impedance networkin the signaling system 100. Consequently, by coupling the referenceoutput terminal 134 of transmitter 102 and the reference input terminal138 of receiver 104 to this low impedance reference node (the powerplane) 130, a stable, consistent reference level may be ensured betweenvarious components of the signaling system, including transmitter 102and receiver 104.

Another advantage of signaling system 100 is that, as discussed belowwith reference to FIG. 1C, a signal current sourced by transmitter 102flows over signaling line 132 to receiver 104 and then returns totransmitter 102 on power plane 130 via a path substantially parallel tothe signaling line 132. Thus, the return current may flow anti-parallelto the signal current, and the signal current and signal-returnconnections at the chip level are essentially differential.Consequently, noise and cross-talk that would otherwise be generated byreturn current flow that is not parallel to the signal current flow canbe avoided. Altogether, the various benefits of the above-describedsignaling arrangement enable construction of a high-speed,ultra-low-power single-ended signaling system that requires on the orderof 1 mW of signaling energy from the power supply—roughly 10-20 timeslower than that of a conventional single-ended signaling system.

FIG. 1B illustrates an embodiment where a ground plane is used as powersupply rail 130, so that V1=+Vs and V2=−Vs. Voltage source(s) 140 isshown as comprised of a voltage source 141 that provides +Vs and avoltage source 142 that provides −Vs. Transmitter 102 may includesignal-controlled switching elements 108, 110 each coupled between acommon drive node and respective signal supply voltages, +Vs and −Vs.Transmitter 102 further includes a resistive element 114 coupled betweenthe common drive node and terminal 133 to act as a series sourcetermination (e.g., to match a characteristic impedance, Z0, of thesignal line). Thus the transmitter's internal Z0 impedance 114 and theimpedance of the line 132, terminated at Z0 by termination impedance116, form a voltage divider that splits the two supply voltages, so thatthe signaling voltage of signal 120 on line 132 toggles between about+Vs/2 and −Vs/2, as shown in FIG. 1B. In some embodiments, the functionsof switches 108, 110 and resistor 114 may be combined in a singledevice, for example an MOS field effect transistor (MOSFET) operating inthe linear or “triode” region of operation; in such embodiments, thelinear relationship of the voltage between the drain and sourceterminals of the MOSFET and the current flowing between drain and sourcecauses the device to operate in a mode approximating a resistor.

Still referring to FIG. 1B, receiver 104 may include a differentialamplifier or other comparator circuit 112. Amplifier 112 amplifies thetime-varying difference between the ground rail and signal potentials toproduce the received data signal (RData) that corresponds to theoriginally transmitted data 106. As shown, a termination element(depicted as resistive element 116) may be coupled between the inputnodes 136 and 138 to terminate the incoming signal line according to thecharacteristic impedance, Z0.

Referring now to FIG. 1C, a signal current 150 sourced by transmitter102 flows over signaling line 132 to receiver 104 and then returns totransmitter 102 on ground plane 130 via a path that can be madesubstantially parallel to the signaling line 132. Thus, noise andcross-talk that would otherwise be generated by return current flow notparallel to the signal current flow can be reduced or eliminated.

In one embodiment, IC devices 100 a and 100 b include IC chips withinrespective semiconductor packages 184 each having an array of solderballs or other metal contacts that act as terminals for the IC devices.FIG. 1D illustrates IC devices 100 a and 100 b on a PCB 101, accordingto one embodiment. FIG. 1D shows two signaling lines 180, 182 betweencircuits 100 a, 100 b, as a subset of a plurality of signaling linesthat would be present on the PCB 101 between circuits 100 a and 100 b.IC device 100 a includes a plurality of transmitters, includingtransmitters 102 a, 102 b. IC device 100 b includes a plurality ofreceivers, including receivers 104 a, 104 b. IC devices 100 a and 100 binclude a plurality of reference (GND) terminals 164, 168 and 174, 178,respectively, that are coupled to a power plane (e.g. ground plane) 131of the PCB 101. The power plane 131 of the PCB 101 can be a solid metallayer in the PCB 101, a metal layer with windows or holes, or it caninclude a plurality of conducting lines each coupled to a same powersupply voltage. Although FIG. 1D shows that power plane 131 is betweenIC devices 100 a and 100 b, in practice, power plane 131 may extendunder IC devices 100 a and 100 b.

IC devices 100 a and 100 b further include a plurality of signalingterminals 162, 166 and 170, 174, respectively. To facilitate routing ofthe signaling lines, the solder balls acting as or coupled to thesignaling terminals 162, 166 and 170, 174 can be placed closer to theedge of the respective semiconductor packages 184 than the solder ballsacting as or coupled to the reference (e.g., GND) terminals 164, 172 and168, 178. For example, in the IC packaging 184 of circuit 100 b (asshown by the insert in FIG. 1D), the signaling terminals 170, 174 areplaced as outer balls of the IC packaging 184 to facilitate easyconnection of the balls to the signaling lines 180 and 182 outside theIC package. On the other hand, the reference terminals 172, 174 can beconnected to the inner balls of the IC packaging 184, because they onlyneed to be coupled to a power plane (e.g., GND plane) within thepackage, which is coupled to the power plane 131 of the PCB 101, towhich the package is soldered or otherwise attached. Such ground packagepins 172, 178 are not as expensive as signal pins 170, 174, since theyare connected directly through vias to a solid conducting plane, therebyrequiring no trace routing. Also, although FIG. 1D illustrates thatseparate reference (e.g., GND) terminals 164, 172 and 168, 178 are usedfor each of the corresponding signaling terminals 162, 170 and 166, 174,it is also possible for two or more of the signal pins 162, 166 and 170,174 to share a common reference terminal, thereby further reducing thenumber of package pins needed for the reference pins.

FIG. 1E illustrates a cutaway view of an IC package including IC device100 a (or IC device 100 b) on a printed circuit board. According to oneembodiment, referring to FIG. 1E, IC device 100 a is mounted on alaminated IC package substrate 200. Package substrate 200 is amulti-layer package substrate having signal wiring conductive layers,including a top layer (e.g., signal trace 202) and a bottom layer (e.g.,solder-ball pads 206 a and 206 b). Package substrate may also includeadditional conducting layers separated from each other and from the topand bottom layers by insulating layers 204. The additional layers mayused for power delivery and signal return current conduction. Forexample, layer 201 in FIG. 1E may be a signal return (and/or reference)plane for the previously described single-ended signaling system, whilelayer 205 may be a plane used to connect to some other power supplyvoltage. In one embodiment, reference plane 201 corresponds to theground reference plane 130 of FIG. 1B (i.e., coupled to an externalground reference) and is sometimes referred to as ground plane 201below. In alternative embodiments, reference plane 201 may be coupled toanother DC reference or even a time-varying reference.

Still referring to FIG. 1E, integrated circuit package 200 may beattached both electrically and mechanically to printed wiring board 190via an array of solder balls, including solder balls 207 a and 207 b.Printed wiring board 200 may be constructed similarly or identically tolaminated package substrate 200, with surface conducting layers, such aslayer 191, inner conducting planes, such as plane 194, and perhapsadditional internal signal layers and power supply planes, that areseparated from each other by insulating layers 193.

Still referring to FIG. 1E, IC device 100 a includes one or moreinstances of ground-referenced signaling receivers and transmitters,such as 102 and 104′ in FIG. 1A. The output pads of these units(corresponding to 133 and 134 in FIG. 1A) are connected to conductingtraces and planes on package 200 via wire bonds 188 and 189,respectively. In FIG. 1E, the signal terminal 133 is connected topackage trace 202 via wire-bond 188. Signal current, denoted by thedashed lines, flows from the chip 100 a, through wire-bond 188 topackage trace 202, thence to through-package via 203, where it iscarried to the bottom layer of the package to solder-ball pad 206 a.Signal current further flows through solder ball 207 a to printed wiringboard solder-ball pad and signal trace 191, and from there to anotherpackaged integrated circuit such as IC device 100 b. Return current(also shown by a dashed line) flows back from the second packagedintegrated circuit over a power supply plane 194, to via 195, where itis conducted up to solder-ball pad 192, through solder ball 207 b,through solder-ball pad 206 b and via 208 to the on-package conductingground plane 201. The electromagnetic interaction between the signalcurrent and this return current will tend to cause the return current toflow in plane 201 in a manner anti-parallel to the signal current insignal trace 202 (i.e., reverse-direction current flow in a pathparallel to the signal trace 202), finally reaching integrated circuit100 a through wire bond 189. In one embodiment, the solder balls 207a/207 b, solder-ball pads 206 a/206 b, and through-package vias 203, 208that carry the signal current and return current are disposed in closeproximity. The close proximity of the solder balls, solder-ball pads,and through-package vias that carry the signal current and returncurrent help avoid large series inductances from accumulating in theoverall signal path, and thus further reduces or eliminates cross-talkwith neighboring signals and their return currents.

FIG. 1E is shown merely by way of example, as there are many other waysof implementing an integrated circuit package and printed wiring boards,including, but not limited to “flip-chip” (C4) construction,package-on-package (POP), and other packaging constructions. Althoughthe technology used and the configuration of the package may vary, thefollowing features should preferably be maintained for optimalperformance: (1) the use of a common, low-impedance power supplyconducting plane (e.g., power supply conducting plane) that carries allreturn currents from one or more supply-reference single-ended signalingcomponents (i.e., transmitters and receivers); and (2) provision for aseparate path from the conducting plane to the reference terminals ofboth transmitters and receivers (e.g., pins 134 and 138 in FIG. 1A),which path may include, for example and without limitation, board andpackage conducting planes, solder balls, through-package andthrough-board vias, wire bonds, and C4 solder bumps. Furthermore, to theextend the extent permitted by the packaging construction technique, thesignaling system may also include an arrangement that forces the signalreturn current to flow anti-parallel to the corresponding signalcurrent, while not overlapping with the signal current return path ofother signaling components.

FIG. 1F illustrates yet another packaging embodiment, similar to theembodiment of FIG. 1D (and including all the variants mentioned above),but in which the ground plane 201 extends beneath the IC die 100 a. Inthe embodiment shown, the IC die 100 a is insulated from the groundplane by a relatively thin layer of epoxy 197. Other insulatingmaterials may be used in alternative embodiments. This approachgenerally provides all the benefits of the arrangement of FIG. 1E whilemaintaining the presence of the ground plane 201 beneath the IC die andthus providing potentially superior shielding and noise suppression.

FIG. 2A illustrates one embodiment of the voltage source(s) 140. Asshown in FIG. 2A, voltage source(s) 140 includes charge pumps 212 and214 for generating the signal supply voltages +Vs and −Vs, respectively,and a charge pump regulator 210. Regulator 210 is coupled to the signalsupply voltages +Vs and −Vs, and receives a reference voltage Vref thatrepresents a desired voltage level for Vs. It controls or regulates thecharge pumps 212 and 214 using control signals ϕ2 b and ϕ2 a,respectively. Charge pumps 212 and 214 are each coupled between firstand second power supply voltages (e.g., Vdd and GND), and are configuredto draw current from the power supply to provide the signal supplyvoltages +Vs and −Vs, and to maintain the signal supply voltages +Vs and−Vs at desired levels in response to control signal ϕ1 and to controlsignals ϕ2 b and ϕ2 a, respectively.

FIG. 2B illustrates an embodiment of regulator 210. As shown in FIG. 2B,regulator 210 receives a pair of non-overlapping clocks ϕ1 and ϕ2 (or,it may include a clock signal source 220 that generates the pair ofnon-overlapping clocks ϕ1 and ϕ2). A pair of equally sized resistors 216and 218 divides a voltage difference between +Vs and −Vs to produce avoltage Vcm (a common-mode voltage between the positive and negativesignal supply voltages). The signal supply voltages +Vs and −Vs arenominally symmetric about GND. A digital or “bang-bang” control loopincluding sense amplifier/latch 224 amplifies the difference (error)voltage between Vcm and GND. When the voltage on Vcm is higher than thevoltage on GND, a signal on line 228 is asserted, and during thefollowing ϕ2 interval, AND gate 234 outputs signal ϕ2 a that trackssignal ϕ2 (i.e., signal ϕ2 a is asserted if signal ϕ2 is asserted). Onthe other hand, if the voltage on Vcm is lower than the voltage on GND,sense amplifier/latch 224 amplifies this difference and de-asserts thesignal on line 228, such that ϕ2 a is not asserted.

Likewise, digital or “bang-bang” control loop including senseamplifier/latch 222 that amplifies the difference (error) voltagebetween +Vs and Vref. If the voltage on +Vs is lower than the voltageVref, a signal on line 226 is asserted, and during the following ϕ2interval, AND gate 232 outputs signal ϕ2 b that tracks signal ϕ2 (i.e.,signal ϕ2 b is asserted if signal ϕ2 is asserted). On the other hand, ifthe voltage +Vs is higher than the voltage Vref, sense amplifier/latch222 amplifies this difference and de-asserts the signal on line 226,such that ϕ2 b is not asserted. By asserting and deasserting ϕ2 a and ϕ2b based on comparison of Vcm with GND and +Vs with Vref, respectively,the signal supply voltages +Vs and −Vs can be maintained at the desiredlevels, as discussed below.

FIG. 2C illustrates another embodiment of regulator 210. Here, thesignal supply voltage +Vs is fed back into comparator 242 for comparisonwith the reference voltage Vref. The output 246 of comparator 242 isasserted if +Vs is smaller than Vref. Thus, the output ϕ2 b of AND gate252 follows the clock signal ϕ2 when +Vs is lower than Vref and is notasserted otherwise. Likewise, the signal supply voltage −Vs is fed backinto window comparator 244 for comparison with the reference voltageVref. The output 248 of window comparator 244 is asserted if thedifference between GND and −Vs is smaller than the difference betweenVref and ground (i.e., GND −(−Vs)<Vref −GND, or Vs is smaller thanVref). Thus, the output ϕ2 a of AND gate 254 follows the clock signal ϕ2when the difference between GND and −Vs is smaller than the differencebetween Vref and GND. Otherwise, it is not asserted.

FIGS. 2D and 2E illustrate embodiments of charge pumps 212 and 214,respectively. Referring to FIG. 2D, charge pump 212 includes a switchingdevice 262 (shown as a P-type metal-on-semiconductor (MOS) field effecttransistor (PFET)) that is turned on or off according to clock signalϕ1, and a switching device 264 (shown as an N-type MOS field effecttransistor (NFET)) that is turned on or off according to clock signal ϕ2b. Charge pump 212 further includes a capacitor 266 and a capacitor 268.Capacitor 266 has a much smaller capacitance than that of capacitor 268and is therefore sometimes referred to as Csmall, while capacitor 268 issometimes referred to as Cbig. Cbig 268 is coupled between +Vs and GND,so the voltage across capacitor 268 is the signal supply voltage +Vs.

FIG. 2F is timing chart showing signals ϕ1, ϕ2, ϕ2 b, the voltage acrosscapacitor 266 (V_Csmall), and the voltage across capacitor 268 (+Vs) incomparison with Vref during a series of time periods after IC deviceincluding the charge pumps is turned on. As shown in FIG. 2F, duringtime period t1, when ϕ1 is asserted, switch 262 is turned on, allowingcharge to flow from Vdd to capacitor 266, so that a voltage acrosscapacitor 266, V_Csmall, increases to a level between Vdd and GND. Thevoltage across Cbig is lower than Vref so ϕ2 b is tracking ϕ2. Duringtime period t2, when ϕ1 is de-asserted while ϕ2 and thus ϕ2 b isasserted, switch 262 is turned off while switch 264 is turned on, sothat stored charge in Csmall is shared with Cbig and the voltage acrosscapacitor 268, +Vs, increases while V_Csmall decreases.

Since the capacitance of Csmall is much smaller than the capacitance ofCbig, and the increase in voltage across Cbig during time period t2 ismuch smaller than the decrease in voltage across Csmall, so the voltageacross Cbig at the end of time period t2 is slightly higher than thevoltage across Cbig at the start of time period t2. The above chargingof Csmall and subsequent charging of Cbig may repeat in response tosignals ϕ1 and ϕ2 b, until the signal supply voltage +Vs reaches or ishigher than Vref at the end of time period t3. As discussed above, aslong as +Vs remains at or above Vref during time periods t4 and t4, ϕ2 bis not asserted, meaning switch 264 does not turn on and Cbig does notget further charged by Csmall. When +Vs drops below Vref during timeperiod t6, ϕ2 b follows ϕ2 again in time period t7, and charges fromCsmall will be shared with Cbig until +Vs is higher than Vref again.

The signal and voltage curves shown in FIG. 2F are for illustrativepurposes only and are not to replicate or scale with signals or voltagesin a real IC device. In some embodiments, the voltage between Vdd andGND is much higher than Vref. For example, Vdd may be about 1 volt,while Vref is about 0.1 volt. In such case, if the charge pumps run at afrequency of, for example, 1 GHz, data is being driven out at, forexample, 4 Gbps, and the impedance of transmitter 102 (thus theimpedance of the load) is, for example, 50 ohms, Csmall can be about0.75 pF to about 1.25 pF while Cbig can be about 50 pF to about 250 pF.

On the other hand, charge pump 214 includes switches 272, 282 that areturned on or off according to clock signal ϕ1, and switches 274, 284that are turned on or off according to clock signal ϕ2 a. Charge pump214 further includes a capacitor Csmall 276 and a capacitor Cbig 278.Cbig 278 is coupled between GND and −Vs so the voltage across Cbig 278is the signal supply voltage −Vs. Switches 272, 282 are turned on whenclock signal ϕ1 is asserted so that charge is extracted from powersupply Vdd via switch 272 and 282 and stored in capacitor Csmall 212.Switches 274 and 284 are turned on when signal ϕ2 a is asserted andsignal ϕ1 is de-asserted, so charges stored in Csmall is pumped intoCbig. This process repeats until the common mode voltage Vcm is lowerthan GND, or the difference between GND and Vs is larger than thedifference between Vref and GND. As discussed above in association withFIGS. 2B and 2C, as long as the common mode voltage Vcm remains lowerthan GND, or the difference between GND and Vs remains larger than thedifference between Vref and GND, ϕ2 a remains de-asserted and no chargeis pumped from Csmall 276 to Cbig 278 to further increase the voltageacross Cbig 278. When the common mode voltage Vcm becomes higher thanGND, or the difference between GND and Vs becomes smaller than Vref, ϕ2a follows ϕ2 again and Cbig 278 gets charged from Csmall until Vcmbecomes lower than GND, or the difference between GND and Vs becomeslarger than the difference between Vref and GND again.

FIGS. 2D and 2E shows that switches 262 and 272 are implemented usingP-type metal-oxide-semiconductor field effect transistors (P-MOSFET orPFET), while switches 264, 274, 282 and 284 are implemented using N-typeMOSFETs (NFET). PFET 262 or 272 is turned on when ϕ1 is asserted byvirtue of inverter 260 or 270, respectively, which inverts the sense ofϕ1 to drive the gate terminal of PFET 262 or 272. Those skilled in theart will easily understand that other types of switches can be used inplace of the MOSFETs. The capacitors 266, 268, 276, and 278 may beimplemented using MOS capacitors, but other types of capacitors canalternatively be employed.

The embodiments of FIGS. 2A through 2F are shown by way of examples.There are many other possible embodiments of on-chip voltage sources orregulators. For example, some embodiments may have additional Csmallcapacitors and additional switches that charge these Csmall capacitorsin a series circuit arrangement and discharge the Csmall capacitors in aparallel arrangement, thereby providing higher overall efficiency thanthat of the simple charge pumps of FIGS. 2D and 2E. In a furtherexample, instead of using clocks to gate the MOSFETs on and off in thecharge pumps, other type of switches that respond to othercharacteristics of control signals such as frequency, amplitude or dutycycle of the control signals may be used. In yet another example,switched capacitors may be used in place of the switch and capacitorcombinations in the charge pumps. In still other examples, otherregulating means such as a linear regulator or switching regulator maybe used to control the signal supply voltages.

In one embodiment, charge pumps 212 and 214 are implemented withcomponents having substantially the same efficiency. In the embodimentshown in FIG. 2A, charge pump 212 draws current from Vdd whentransmitter 102 is transmitting “1”s while charge pump 214 draws currentfrom Vdd when transmitter 102 is transmitting “0” s. Thus, assuming thatthe transmitter 102 has matching impedance with the transmission lines132, 130, the transmitter 102 draws approximately the same amount ofsupply current from the power supply regardless of which way thetransmitter 102 is driving the current (i.e., sourcing current fromtransmitter 102 to receiver 104, or sinking current from receiver 104 totransmitter 102). Thus, to the extent that charge pumps 212 and 214 haveabout the same efficiency, the complementary configuration of thevoltage source(s) 140 ensures that supply current variation in signalingsystem 100 is near zero, thereby reducing or eliminating simultaneousswitching noise (SSO) inherent in conventional single-ended signalingschemes.

FIGS. 3A, 3B, and 3C illustrate several alternative embodiments for thereceiver amplifiers used in the data transmission system 100 of FIGS.1A, through 1D, when the GND is chosen as the reference voltage,according to one embodiment. FIG. 3A illustrates a PMOS amplifier 302that can be used in some embodiments of receiver 104, coupled to the twoterminals 136, 138 corresponding to the signaling voltage and GNDreference voltage, respectively. This amplifier is a conventional PMOSdifferential amplifier with resistor loads. Numerous alternativeembodiments of a PMOS differential amplifier are possible. FIG. 3Billustrates one embodiment of a common gate amplifier 304 that can beused in receiver 104 in other embodiments, coupled to the two terminals136, 138 corresponding to the signaling voltage and GND referencevoltage, respectively. The common gate amplifier 404 in FIG. 3B isimplemented with NMOS transistors and thus may have a highergain-bandwidth product than the PMOS amplifier 402 of FIG. 3A.

FIG. 3C is a common-gate amplifier 306 according to another alternativeembodiment. Current source 906 establishes a reference current in NFET914, thereby generating gate reference voltage Vcas. NFETs 910 and 912,which are the amplifying transconductances in amplifier 306, are drawnat the same shape factor (width/length) as 914 so that they carry aboutthe same current as 914, when their inputs, driven into their sourceterminals, attached to pins 136 and 138, are at substantially the samepotential, since in this case the gate-to-source voltages (Vgs) for allthree transistors are substantially equal and the shape factors areidentical. The data signal, attached to pin 136, is compared inamplifier 306 with the signal reference voltage, input on pin 138 fromthe power supply GND plane 130. The current from line to GND develops avoltage across termination resistor 920; this voltage swings from about+Vs/2 to −Vs/2. When the voltage on line (pin 136) is higher than thevoltage on GND (pin 138) NFET 910's Vgs is less than Vgs at NFET 912.Therefore, less current flows through 910 and through 912, and thevoltage drop through load resistor 902 is smaller than the voltage dropthrough load resistor 904. This causes the voltage on outH to rise abovethe voltage on outL. On the other hand, if the voltage on 136 is lowerthan the voltage on 138, then NFET 910's Vgs is greater than Vgs forNFET 912. Consequently NFET 910 carries more current than 912, and thevoltage across 902 is larger than the voltage across 904. In this case,the voltage on outH is less than the voltage on outL. Under thesecircumstances the current driven back into line 132 (pin 136) by NFET910 is larger than the current driven into the line 132 by 910 when thevoltage on line exceeds the voltage on GND. To prevent this currentunbalance from introducing an unwanted voltage offset in amplifier 306,a compensating resistor 922, RC, is used to return some of the currentto the negative power supply −Vs.

Still referring to FIG. 3C, the magnitude of the voltage between outHand outL is higher than the magnitude of the voltage between line (pin136) and GND (pin 138) thanks to the gain of amplifier 306; the gain of306 is about Gm×RL, where Gm is the transconductance of NFETs 910 and912, and RL is the resistance of load resistances 902 and 904.

Still referring to FIG. 3C, it will be clear to those skilled in theart, that the terminating impedance presented across pins 136 and 138 isthe parallel combination of the terminating resistor RT 920, thecompensating resistor RC 922, and the input impedance of the amplifier,which is about 1/Gm, where Gm is the transconductance of transistors 910and 912. This parallel combination of resistances should be adjusted toabout the characteristic impedance of the transmission line formed byline 132 and GND to avoid reflections in the transmission line.

The foregoing embodiments of input amplifiers are shown by way ofexamples. Many alternative embodiments of input amplifiers can beemployed in receiver 104 or 104′ in system 100.

FIG. 4A illustrate a data transmission system employing GND referencedsingle-ended signaling with ESD protection, according to one embodiment.The data transmission system 400 includes circuit 100 a including thetransmitter 102 and circuit 100 b including the receiver 104, andemploys the single-ended signaling scheme as described above using thesignaling line 132 and GND reference line 130. Voltage sources such ascharge pumps (CP) 141, 142 supply the positive and negative signalsupply voltages V_(S1) and −V_(S1), and either switch 108 or switch 110is turned on to source or sink line current on the signaling line 132responsive to input data 106, as explained above. The signaling voltageis developed across resistor 116 and detected by input amplifier 112 torecover the input data 106.

The data transmission system 400 additionally includes ESD(Electrostatic Discharge) protection devices 402, 404. In oneembodiment, each ESD protection device 402, 404 is comprised of a pairof cross-coupled diodes connected between the signaling voltage 132 andthe GND reference voltage 130 of the single-ended signaling system, withESD protection device 402 connected on the transmitter side 102 and theESD protection device 404 connected on the receiver side 104. Note thatthe cross-coupled diodes in ESD protection device 404 are coupledbetween the inputs of the input amplifier 112 in parallel with resistor116. The ESD protection devices 402, 404 have the benefit of reducingthe input capacitance of the I/O pins 133, 134 and 136, 138 andprotecting the pins 133, 134 and 136, 138 from electrostatic discharge.Also, as shown in FIG. 4B, the allowed voltage swing 408 (V_(L)=−50 mVto V_(H)=+50 mV) of the signaling voltage 132 should be within themargins 416, 418 between the regions 406, 408 in which diodes 402, 404become forward biased. Symmetrically connecting the diodes 402, 404 tothe GND signaling reference voltage 130 allows the voltage swing of thesignaling voltage on line 132 to be larger than for asymmetricconnection, because the margins 416, 418 between the regions 406, 410also become symmetrical with respect to the GND reference voltage 130and thus no margins 416, 418 between the two regions 406, 410 arewasted.

FIG. 5A illustrates a configurable data transmission system configuredin single-ended signaling mode, according to one embodiment. The datatransmission system 500 includes circuits 500 a, 500 b. Circuit 500 aincludes a transmitter 502 and circuit 500 b includes a receiver 504.Transmitter 502 and receiver 504 and employs the ground-referencedsingle-ended signaling as described above using the signaling line 132and GND reference line 130. Charge pumps (CP) 140, 142 supply thepositive and negative signal supply voltages V_(S1) and −V_(S1) forgenerating the signaling voltage on line 132, and either switch 108 orswitch 110 is turned on to source or sink line current on the signalingline 132 responsive to input data 106 as explained above. The signalingvoltage is detected across resistor 116 and detected by input amplifier112 to recover the input data 106. ESD protection devices 402, 404 eachcomprised of a pair of cross-coupled diodes are also connected betweenthe terminals 133, 134 and between the terminals 136, 138, respectively.

Circuit 500 a additionally includes terminal 532 and ESD protectiondevice 522, and transmitter 502 additionally includes switches 508, 510and resistor 544. Circuit 500 b additionally includes ESD protectiondevice 524 and terminal 536, and the receiver 504 additionally includesswitches 512, 514, 516, 518. An additional transmission line 552 iscoupled between terminal 532 and terminal 536. Terminals 133, 134, 532form one configurable unit (set) of pins of the transmitter 502 among anumber of output pins that would be present on the IC including thetransmitter 502. Similarly, terminals 136, 138, 536 form oneconfigurable unit (set) of pins of the receiver 504 among a number ofoutput pins that would be present on the IC including the receiver 504.However, when the data transmission system 500 is used in single-endedsignaling mode, terminals 532, 536, line 552, ESD protection devices522, 524 are not used, because switch 518 is turned off, disconnectingthe input amplifier 112 from terminal 536 and line 552. Also, switch 512and switch 518 are turned on, connecting terminals 136 and 138 to theinputs of amplifier 112. Switch 514 is a dummy transistor for providingmatched loading corresponding to switch 518, but is also turned off. Onthe transmitter side 502, switches 508, 510 and resistor 544 are notused because they are connected to line 552 that is disconnected frominput amplifier 512. Thus, the data transmission system 500 isconfigured for single-ended signaling mode, similar to the transmissionsystem 400 of FIG. 4, using signaling line 132 and the GND referenceline 130.

FIG. 5B illustrates a configurable data transmission system configuredin differential signaling mode, according to one embodiment. Thecomponents and structure of the data transmission system 500 shown inFIG. 5B are the same as those of the data transmission system 500 shownin FIG. 5A. However, the data transmission system 500 in FIG. 5B isconfigured to operate in differential signaling mode. Differentialsignaling transmits data by means of two complementary signals sent ontwo separate wires. Here, lines 132 and 552 are used as thecomplementary (positive and negative) signaling lines for differentialsignaling, and the GND reference line 130 is not used. On the receiverside 504, switches 512, 518 are turned on to connect terminals 136, 536(and signaling lines 132, 552) to input amplifier 112, respectively, butswitch 516 is turned off to disconnect terminal 138 (and the GNDreference line 130) from the input amplifier 112. Switch 514 is alsoturned off. On the transmitter side 502, both pairs of switches 108, 110and 508, 510 are used to generate the complementary differential signalsto be sent over transmission lines 132, 552. Switches 108, 110 areconnected to signaling line 132 via resistor 114 to generate thepositive differential signal (Out+) on line 132, and switches 508, 510are connected to signaling line 552 via resistor 544 to generate thenegative differential signal (Out−). Further, both ESD protectiondevices 402, 522 are used on the transmitter side 502 and both ESDprotection devices 404, 524 are used on the receiver side 504. Thus, thedata transmission system 500 is configured for differential signalingmode, using both signaling lines 132, 552 and terminals 133, 532 ontransmitter side 502 and terminals 136, 536 on the receiver side 504.

The data transmission system described in FIGS. 5A and 5B isconfigurable to a single-ended signaling system or a differentialsignaling system in a convenient manner, simply by turning on or offswitches 516, 518. For example, by adding a pin (e.g., pin 536) forevery pair of pins (pins 136, 138) on the receiver side 504, a signalinginterface that is configurable and versatile with high signal integritycan be achieved.

Alternatively, no additional pins are required. In single-endedsignaling mode each pair of pins are line pin and reference or GND pin,respectively. In differential signaling mode they are lineP pin andlineN pin, respectively. The lineN half of the transmitter is turned offin single-ended signaling mode, and a large NFET switch can connect whatwas lineN to internal Vss. A dummy switch on lineP can be added to avoidimbalances. Both terminals would have cross-coupled diodes returned tothe GND rail. At the receiver, no switches are required. The lineN inputcan just be connected to GND. Thus, a configurable signal transmissionsystem similar to that described in FIGS. 5A and 5B but without theextra pins 536 and 532 can be formed.

It should be noted that the various integrated circuits, dice andpackages disclosed herein may be described using computer aided designtools and expressed (or represented), as data and/or instructionsembodied in various computer-readable media, in terms of theirbehavioral, register transfer, logic component, transistor layoutgeometries, and/or other characteristics.

When received within a computer system via one or more computer-readablemedia, such data and/or instruction-based expressions of the abovedescribed circuits may be processed by a processing entity (e.g., one ormore processors) within the computer system in conjunction withexecution of one or more other computer programs including, withoutlimitation, net-list generation programs, place and route programs andthe like, to generate a representation or image of a physicalmanifestation of such circuits. Such representation or image maythereafter be used in device fabrication, for example, by enablinggeneration of one or more masks that are used to form various componentsof the circuits in a device fabrication process.

In the foregoing description and in the accompanying drawings, specificterminology and drawing symbols have been set forth to provide athorough understanding of the present invention. In some instances, theterminology and symbols may imply specific details that are not requiredto practice the invention. For example, any of the specific numbers ofbits, signal path widths, signaling or operating frequencies, componentcircuits or devices and the like may be different from those describedabove in alternative embodiments. In other instances, well-knowncircuits and devices are shown in block diagram form to avoid obscuringthe present invention unnecessarily. Additionally, the interconnectionbetween circuit elements or blocks may be shown as buses or as singlesignal lines. Each of the buses may alternatively be a single signalline, and each of the single signal lines may alternatively be buses.Signals and signaling paths shown or described as being single-ended mayalso be differential, and vice-versa. A signal driving circuit is saidto “output” a signal to a signal receiving circuit when the signaldriving circuit asserts (or deasserts, if explicitly stated or indicatedby context) the signal on a signal line coupled between the signaldriving and signal receiving circuits. The term “coupled” is used hereinto express a direct connection as well as a connection through one ormore intervening circuits or structures. Integrated circuit device“programming” may include, for example and without limitation, loading acontrol value into a register or other storage circuit within the devicein response to a host instruction and thus controlling an operationalaspect of the device, establishing a device configuration or controllingan operational aspect of the device through a one-time programmingoperation (e.g., blowing fuses within a configuration circuit duringdevice production), and/or connecting one or more selected pins or othercontact structures of the device to reference voltage lines (alsoreferred to as strapping) to establish a particular device configurationor operation aspect of the device. The terms “exemplary” and“embodiment” are used to express an example, not a preference orrequirement.

While the invention has been described with reference to specificembodiments thereof, various modifications and changes may be madethereto without departing from the broader spirit and scope. Forexample, features or aspects of any of the embodiments may be applied,at least where practicable, in combination with any other of theembodiments or in place of counterpart features or aspects thereof.Accordingly, the specification and drawings are to be regarded in anillustrative rather than a restrictive sense.

1. (canceled)
 2. An integrated circuit to be coupled to a power supply,comprising: first and second terminals, the second terminal to be at apower supply voltage after being coupled to the power supply; atransmitter configured to transmit a signal via the first terminal, thesignal having a voltage level that is referenced to the power supplyvoltage and that swings substantially symmetrically above and below thepower supply voltage; a first charge pump drawing a first supply currentfrom the power supply while the voltage level of the signal is at ornear a first voltage; and a second charge pump drawing a second currentfrom the power supply while the voltage level of the signal is at ornear a second voltage, the first voltage and the second voltage beingsubstantially symmetrical with respect to the power supply voltage andthe second current being substantially equal to the first current. 3.The integrated circuit of claim 2, wherein the second terminal is partof a signal return path for the signal.
 4. The integrated circuit ofclaim 2, wherein the power supply voltage is a ground voltage.
 5. Theintegrated circuit of claim 2, wherein the power supply voltage is otherthan a ground voltage.
 6. The integrated circuit of claim 2, wherein thetransmitter transmits the signal while the first charge pump and thesecond charge pump draw a total power supply current from the powersupply that is substantially independent of the voltage level of thesignal.
 7. The integrated circuit of claim 2, further comprising: areceiver amplifier including a first input and a second input coupled tothe first terminal and the second terminal, respectively, the receiveramplifier configured to detect a voltage difference between a signalingvoltage at the first terminal with reference to the power supply voltageat the second terminal.
 8. The integrated circuit of claim 7, whereinthe first input is connected to a first source of a first N-channelfield effect transistor (NFET) and the second input is connected to asecond source of a second NFET.
 9. An integrated circuit, comprising: afirst terminal; a second terminal to receive a power supply voltage; afirst charge pump to charge a first capacitor to a voltage differentialacross the first capacitor; a second charge pump to charge a secondcapacitor to the voltage differential across the second capacitor;signal controlled switching elements to apply, across the first terminaland the second terminal, the voltage differential across the firstcapacitor for a first set of time periods and apply the voltagedifferential across the second capacitor for a second set of timeperiods to generate a signal between the first terminal and the secondterminal, the signal having a voltage level that is referenced to thepower supply voltage and that swings substantially symmetrically aboveand below the power supply voltage, the first set of time periods andthe second set of time periods being substantially non-overlapping. 10.The integrated circuit of claim 9, wherein the second terminal is partof a signal return path for the signal.
 11. The integrated circuit ofclaim 9, wherein the power supply voltage is a ground voltage.
 12. Theintegrated circuit of claim 9, wherein the power supply voltage is otherthan a ground voltage.
 13. The integrated circuit of claim 9, whereinthe integrated circuit transmits the signal while drawing a power supplycurrent from the power supply that is substantially independent of thevoltage level of the signal.
 14. The integrated circuit of claim 9,further comprising: a receiver amplifier including a first input and asecond input coupled to the first terminal and the second terminal,respectively, the receiver amplifier configured to detect a voltagedifference between a signaling voltage at the first terminal withreference to the power supply voltage at the second terminal.
 15. Theintegrated circuit of claim 14, wherein the first input is connected toa first source of a first N-channel field effect transistor (NFET) andthe second input is connected to a second source of a second NFET. 16.The integrated circuit of claim 14, wherein the receiver amplifier has acommon-gate configuration.
 17. An integrated circuit, comprising: afirst terminal; a second terminal to receive a power supply voltage; aplurality of charge pumps to charge a respective plurality of capacitorsto a voltage differential level across the plurality of capacitors;signal controlled switching elements to apply, across the first terminaland the second terminal, the voltage differential across respective onesof the plurality of capacitors for corresponding sets of time periods,the applied voltage differentials to generate a signal between the firstterminal and the second terminal, the signal having a voltage level thatis referenced to the power supply voltage and that swings substantiallysymmetrically above and below the power supply voltage, the sets of timeperiods being substantially non-overlapping.
 18. The integrated circuitof claim 17, further comprising: a receiver amplifier including a firstinput and a second input coupled to the first terminal and the secondterminal, respectively, the receiver amplifier configured to detect avoltage difference between a signaling voltage at the first terminalwith reference to the power supply voltage at the second terminal. 19.The integrated circuit of claim 17, wherein the first input is connectedto a first source of a first N-channel field effect transistor (NFET)and the second input is connected to a second source of a second NFET.20. The integrated circuit of claim 19, wherein the receiver amplifierhas a common-gate configuration.
 21. The integrated circuit of claim 20,wherein the power supply voltage is a ground voltage.